1. Field of the Invention
The invention relates to an impression cylinder of a sheet-fed offset printing machine for first form and perfector printing, the impression cylinder having a multiple diameter, i.e. a diameter which is, for example, double or triple the given conventional diameter of a plate or blanket cylinder of such a printing machine, and, more particularly, to such an impression cylinder having a packing or dressing provided on the outer cylindrical surface of the cylinder for carrying a sheet on a previously printed side thereof.
2. Description of the Related Art Including Information Disclosed Under 37 C.F.R. 1.97 to 1.99.
Impression cylinders having double the conventional size and having a chromium-plated, previously ground outer cylindrical surface have become known heretofore. The additive effect of inequalities in the shape of the two halves of the ground cylinder may cause slurring or double printing while the machine is running and, during perfecting, also a redistribution of the previously printed ink so that the impression cylinder has to be washed more often, thereby increasing the consumption of washing solution detrimental to the environment. This has a negative effect upon the quality of the printed product.
Also known heretofore are double-sized impression cylinders which have been subjected to blasting before being chromium-plated and, nevertheless, have relatively rough surfaces, and an even more inadequate accuracy of shape due to the defects in the impression cylinder which is formed of two unlike halves, so that such impression cylinders tend readily to slurring. As soon as the surface begins to wear, the entire impression cylinder has to be replaced, with resulting high costs for material and assembly as well as downtime in the printshop.
British Patent 1,515,342 discloses an impression cylinder with a single diameter having a nickel foil with a textured surface mounted on the outer cylindrical surface thereof, thus dividing the outer cylindrical surface into raised sheet-supporting areas and depressed non-supporting areas so as to minimize the build-up and accumulation of ink on the outer cylindrical surface when it comes into contact with the freshly printed side of a sheet. Foils serving this purpose have become known heretofore from U.S. Pat. No. 4,327,135.
Impression cylinders with a multiple diameter are more difficult to manufacture than cylinders having the conventional given diameter of plate and blanket cylinders. The multiple-diameter impression cylinders have manufacturing tolerances which add to the tolerances of packing plate thicknesses. The result is commonly referred to as summation faults or errors. This especially occurring faults or errors are slight noncircularity or nonroundness or eccentricity. Such deviations are not able to be fully eliminated even with the best manufacturing processes. They cause a shifting and doubling, respectively, of screen dots which results in ghosting or slurring, both for single-rotation as well as for double-rotation cylinders. Color tones are thereby slightly altered and, accordingly, deviate from nominal color tones.
In single-rotation impression cylinders, color-tone shifting, in the case of slight manufacturing errors, is barely able to be noticed, because it occurs the same for all printings. If the slurring or ghosting becomes too great, a counter-control may be effected with the register device and the error may accordingly be minimized or even entirely eliminated.
On the other hand, in the case of impression cylinders having a double diameter, i.e., with two sheet-supporting surfaces, the screen dot shifts of pairs of successive sheets occur very differently, so that even the smallest color shifts are readily apparent. A screen-dot shift caused by eccentric or non-round rotation of a double-diameter cylinder, for example, cannot be eliminated with the register device because a minimizing of the error in the printed image of one of the sheet-supporting surfaces results in an inverse in the error of the other sheet-supporting surface.